The present invention relates to a plastic concentrate containing pigment and/or filler, for the production of a polyolefin coating compound for photographic support materials. More particularly, the invention relates to the use of a double screw extruder with degassing for the production of photographic pigmented plastic concentrates.
Waterproof photographic support materials comprise a plastic film or base paper with a synthetic resin coating compound applied, preferably to both sides, by an extrusion process. The synthetic resin coating typically comprises a polyolefin which may contain light reflecting white pigments and, if necessary, color pigments, optical brighteners and/or other additives, such as antistatic agents, dispersing agents, anti-oxidants, release agents, etc. Polyolefins suitable for the extrusion coating of photographic base papers enriched with titanium dioxide pigment include polyethylene in its LDPE or HDPE form, ethylene-alpha-olefin-copolymerisate, so-called LLDPE, and polypropylene, as well as mixtures thereof.
The titanium dioxide can be used in its rutile and/or anatase modification and also in an inorganic and/or organic surface treated form. The inorganic surface treatment can be carried out with Al.sub.2 O.sub.3, SiO.sub.2, Mg(OH).sub.2 or zirconium aluminates, and the organic surface treatment can be carried out with organic polysiloxanes, silanes, multifunctional alcohols, alkyl titanates, alkanolamines, etc. In addition to titanium dioxide, other pigments or fillers may be used, such as ultramarine, cobalt blue, cobalt violet, carbon black, Al.sub.2 O.sub.3, SiO.sub.2 or surface-treated calcium carbonate.
It is also known that the sharpness of an image is related to the amount of titanium dioxide in the polyolefin layer of the photographic support material. A higher titanium dioxide content in the polyolefin layer provides a higher degree of sharpness of the image.
The complete disintegration of the pigment agglomerates and the homogeneous distribution of the pigment particles are the presupposition for the complete development of the pigment characteristics in the plastic, as well as for trouble-free processing. This is particularly important in the production of thin coatings (25-50.mu.), in which TiO.sub.2 concentrations of up to 20% or more are necessary. In thin polyolefin layers, pigment agglomerates, on the one hand, reduce the light reflection and, on the other hand, lead to the formation of holes and cracks and, finally, to the tearing of the film. A good dispersion of the pigment is therefore necessary.
Thus, for the pigmenting of polyolefins, rather than pure pigments, pigment plastic concentrates have been used in which the pigment is already present in dispersed form. Such concentrates are prepared in a special work operation with the aid of kneading machines, rolling mills or mixers, whereby the increase of shearing forces causes an improvement in the dispersal of the pigment.
In accordance with the present state of the art, plastic concentrates are produced in accordance with various methods (such as for example U.S. Pat. No. 4,650,747; JP 60 75 832). All methods are characterized by the use of auxiliary dispersing agents, for example salts or esters of higher fatty acids, such a stearates, and the use of surface-treated titanium dioxides. In all the prior methods kneading machines and mixers, preferably Banbury mixers, were employed.
The plastic concentrates produced in accordance with the methods stated above are always characterized, despite the use of the auxiliary methods stated above, by a non-homogenous consistency. The pigment particles are presumably not completely covered and, as a result, optimal dispersibility is not ensured.
Polar pigments, such as for example TiO.sub.2, particularly when incorporated into non-polar polymers, such as polyolefins, tend to a pronounced reagglomeration, due to the high surface energy which is imparted to them. This is particularly true with pigment concentrations of greater than 15 weight %. Thus, a limit to the concentration of the pigments may have to be set which may result in the inability to attain desired or necessary concentrations. Furthermore, due to this reagglomeration difficulties arise during the extrusion coating, such as for example excessively great sieve residue, the plugging of the extrusion dies and holes and cracks or crazes in the film.
The kneading machines and mixers used in the state of the art in the production of photographic plastic concentrate master batches as a rule operate in a non-continuous manner and without degassing. These kneading machines and mixers have been preferentially employed up until the present because the processing temperatures are in the range of 120.degree. to 140.degree. C. and thereby the thermal stress on the plastic is not too great. The conventional Banbury mixer, for example, comprises two spiral-shaped blades located in a mixing chamber and which rotate in opposite directions. After the mixing process has been completed (approximately 2 minutes), the chamber is emptied of the resulting master batch.
Additive materials, such as fillers or pigments normally contain moisture, air and/or surface-active agents which lead to a reduction in the quality of the master batch. The chemically bonded residual moisture on the surface of the pigment particles and the moisture in the pores lead, for example, to so-called "water braids" in the extrusion coating. Moreover, the so-called "kneading device oil" which is forced from the outside into the bearings of the kneading blades for lubrication penetrates into the plastic mixture. During the later extrusion coating, the oil evaporates, condenses on the machine parts, drops onto the paper sheet and leads to defects in the polyethylene adhesion on the base paper. All volatile components arising from plastics or plastic mixtures generally impede the faultless covering of the pigment or filling material particles and lead to porous granulate which results in the reduced quality of the surface structure of the later coating. Furthermore, auxiliary dispersing agents which exude out and precipitate on the coating surface, results in difficulties later during the further processing of the coated support material, such as for example dull spots on the surface and reduced adhesion between the support material and light-sensitive emulsions.
It is thus the task of the invention to produce a plastic concentrate containing a pigment and/or filler, by means of which it is possible to produce a faultless coating compound for photographic support materials with a uniform distribution of the pigment or the filler therein, and the coating compound should ensure trouble-free processing.
This task is solved by the use of a continuously operating double screw extruder with degassing zone for the production of plastic concentrates containing pigment and/or fillers. The extruder is preferably one with two screws operating in a synchronous and interlocking manner. Contra-rotating double screw extruders can also be used, however. The degassing can take place in one or several stages.
It has surprisingly been found that in addition to a good degassing of the plastic concentrate, which was to be expected and the elimination of the above-mentioned difficulties which were connected with the same, such as for example "water braiding" or the contamination of the sheet by the kneading device oil and additives which are exuded out, a very good dispersion of the pigment particles without formation of agglomerates will result. This good dispersion also seems to be all the more surprising, since the addition of auxiliary dispersing agents can be dispensed with.
One further surprising effect is the fact that despite fears regarding the high temperatures prevailing in the double screw extruder (250.degree.-300.degree. C.), a master batch with low gels formation can be produced. Indeed, the number of gels formed is nearly one half less than what would be normal in the case of photographic master batches produced by the conventional methods.